How to do a Load Study Desktop Calculation

How to do a load study

Facility managers are often asked to add or relocate equipment. The critical question is: does your electrical system have the capacity to handle it?

This is where an electrical load study comes in. While engineers typically use specialized meters and software, you can attempt a “back-of-the-envelope” desktop calculation if you know the steps. Just be prepared — it’s more complicated than it looks.

An electrical load study evaluates whether your facility’s electrical system can handle additional loads without overloading the service or violating code.

  • NEC (National Electrical Code) Article 220.87 requires using actual demand data to determine capacity before adding loads.
  • NFPA 70B (2023) emphasizes ongoing load assessment as part of a documented Electrical Maintenance Program (EMP).

If you’re new to this topic, we covered the basics in our blog Do You Have the Power for New Equipment? — a great starting point before diving into desktop load study calculations.

The goal is simple: protect your facility from overloads, downtime, and costly non-compliance.

A load study protects your facility from overloads, unplanned downtime, and compliance violations.

Here’s how facility managers might calculate available capacity using only utility data and some math.

Look at the utility bill from your peak demand month — often August for cooling loads. Find the maximum short-interval demand (usually 15 minutes).

If your bill lists kilowatts (kW), convert to kVA using:

kVA = kW Power Factor

For this example, let’s use 800 kVA.

Identify your incoming service rating. Example: 2,000 amps at 480 volts, three-phase.

Use the three-phase formula:

Ipeak = kVA × 1000 √3 × V

Example: (800 × 1000) ÷ (√3 × 480) = 962A

Per NEC rules, continuous loads can only use 80% of service rating.

  • 2,000 A × 80% = 1,600 A usable capacity.
  • Peak load = 962 A.
  • Utilization = 962 ÷ 1,600 = 60%.

What if you want to add a machine that draws 200 A?

  • NEC 220.87 requires: (Peak Demand × 125%) + New Load ≤ Service Rating.
  • (962 A × 1.25) + 200 A = 1,403 A.
  • Compare: 1,403 A ≤ 2,000 A service rating (and 1,600 A continuous limit).

Result: The system can handle it — but you’re closer to the edge.

By code, you must factor in 125% of peak demand before adding new load.

Doing this yourself isn’t as simple as it looks:

  • Complex math: Power factor, kVA vs kW, and three-phase formulas can be confusing.
  • Data gaps: Utility bills may miss short-duration peaks.
  • Hidden overloads: Subpanels and feeders can be maxed out even if the main service looks fine.
  • Compliance: AHJs may not accept manual calculations for permits — they may require 30-day logged data.
  • Risk: One misstep in the math could mean an undersized system and a serious safety hazard.

Ready to Simplify Your Load Study?

Back-of-the-envelope calculations are possible, but they’re overwhelming and risky if done alone. Don’t gamble with your facility’s power capacity. Partner with C&H Electric to ensure your system is ready for new equipment — safely, reliably, and in compliance.

References

  • National Fire Protection Association (NFPA). (2023). NFPA 70B: Standard for electrical equipment maintenance (2023 ed.). NFPA.
  • National Fire Protection Association (NFPA). (2023). NFPA 70: National Electrical Code (NEC), Article 220.87. NFPA.
  • Occupational Safety and Health Administration (OSHA). (2023). 29 CFR 1910 Subpart S: Electrical. U.S. Department of Labor.
  • Electrical Contractor Magazine. (2022). No More Guesswork: Determining Existing Loads.
  • EC&M Magazine. (2023). Ensuring Accuracy in Demand Factors with NEC.